Growth of microorganisms in liquid culture
A colorimeter or light sensor with data logger can be used to measure the rate of growth of
microorganisms like yeast. As a yeast population grows the broth they live in becomes more
cloudy due to the presence of more microorganisms. This increase in turbidness means less
light passes through the culture, so by monitoring the light passing through the culture across a
24 hour period their growth can be estimated.
Equipment list
- Bench disinfectant
- Bunsen burner
- Colorimeter
- 0.5% glucose solution
- Cuvettes
- Measuring cylinders
- Conical flask
- Pipettes
- Yeast culture
- Magnetic stirrer
- Weighing scales
Method
1. Before starting the experiment disinfect the workbench, this helps to prevent contamination
when preparing the yeast culture.
2. Fill a 500 cm3 conical flask with 250 cm3 of the glucose solution and then weigh out 1.25g of
yeast and add it to the flask along with a magnetic stirring flea that enables you to
constantly and evenly stir the culture.
3. Place the flask on the magnetic stirrer base and stopper the flask with some cotton wool
secured with a covering of aluminium foil.
4. The flask can be incubated at room temperature.
Measuring growth: Method 1 - Colorimetry
1. Use a pipette to fill a cuvette with the 0.5% glucose solution and take a colorimetry reading
to set the reference absorbance of the colorimeter to zero.
2. Once the yeast sample has been prepared, use a sterile pipette to transfer 4 cm3 of the
culture into a cuvette and take an absorbance reading, recording the age of the culture in
minutes and its absorbance in a table.
3. Repeat steps 1 and 2 at the following ages of the culture: 30 minutes,1 hour, 90 minutes, 2
hours, 5 hours, 8 hours, 11 hours.
Measuring growth: Method 1 - Light Sensor with Datalogger
1. Set up the apparatus as shown.
2. Leave the datalogger to continuously record absorbance readings for 12 hours.
Graph
The shape of the graph would show the exponential growth of the yeast culture.
